Prior art propeller pumps have been designed according to a hypothesis that is recognized within the technical field of propeller pumps and, among other things, is based on the following. Propeller pumps are designed in such a way that the cross-sectional area of the channel of the propeller pump should, within an as short as possible axial distance, increase from the cross-sectional area (A1) found in the region of the rear edge of the blades of the propeller to as large as possible cross-sectional area (A2) in the region of the rear edge of the guide vanes, and after that increase further to a larger cross-sectional area (A3) in the region of the outlet opening of the channel. This is for minimizing the losses and having as large as possible pressure regain. However, the possibility of minimizing the axial distance is limited by the fact that separation (regions with rearwardly directed flows) arises at too steep increase of the cross-sectional area. The emergence of separation means that the losses increase considerably. In prior art propeller pumps, the knowledge of what degree of cross-sectional area increase is possible without separation arising has been based on inviscous calculations wherein the designers have relied on empirical, so-called diffusion factors to determine whether separation arises in the guide vane passage. These factors were developed by cascade tests in the 1950's. As for the diffuser after the rear edge of the guide vanes to the outlet opening of the channel, one has been reduced to so-called performance charts for annular diffusers.
Below, examples of recognized area relationships according to the above-mentioned hypothesis follow: [A2≈1.4*A1] and [A3≈2.3*A1]. These area relationships are valid for propeller pumps having relatively high specific rotational speeds (nq), for instance within the range of 200-300, which is a measure of how great liquid flow Q can be transported to a certain pressure head H of a propeller pump operating at a nominal rotational speed n, wherein [nq=n*Q(1/2)/H(3/4)]. As a consequence of the fast area increase, such a design involves that a lower flow rate is obtained in the fastest possible manner, and a direct consequence of this has, according to the hypothesis, been considered to be that the losses that arise in the region downstream the upper end of the propeller pump will be minimized.
Propeller pumps designed according to the above-mentioned hypothesis have, however, in a quite opposite way turned out to create large losses and large regions of separation in the channel in the region of the guide vanes and/or in the diffuser downstream the rear edge of the guide vanes and/or in the column pipe downstream the propeller pump. This depends on the diffusion factors being based on two-dimensional experiments that do not take into account e.g., secondary flow and the curvature of the channel. Also the performance charts for diffusers have limitations, as for instance that they presuppose so-called linear end walls (the envelope surface of the pump core and the inner surface of the pump housing) and a uniform flow rate profile into the diffuser, i.e., a uniform flow rate profile along the cross-sectional area taken in the region of the rear edge of the guide vanes.